In recent years, a magnetostrictive element has come to be widely used e.g., in small-stroke reciprocating actuators that actuate an opening/closing valve for a hydraulic circuit or a pneumatic circuit. In this case, the magnetostrictive element is worked into a magnetostrictive rod, and is responsive to a magnetic field applied from an electromagnetic coil for having its longitudinal dimension adjusted to thereby actuate a valve element of the opening/closing valve formed to extend in the direction of expansion of the magnetostrictive element as an object to be actuated by the same, thereby opening and closing the opening/closing valve. However, the magnetostrictive rod suffers from the problem that it varies in longitudinal dimension with a change in temperature due to its characteristic, and therefore the amount of actuation of the valve element varies due to a change in temperature. To solve this problem, there has been proposed an ultra-magnetostrictive actuation assembly in Japanese Laid-Open Patent Publication (Kokai) No. H07-236292, which comprises as shown in FIGS. 1 and 2 of the publication, a temperature-detecting means (8) for detecting a temperature in the vicinity of a ultra-magnetostrictive rod (5), and a magnetic field control means (9) for controlling a magnetic field generated by a solenoid coil (4) based on the temperature detected by the temperature-detecting means (8), and operates to provide control such that the length of the ultra-magnetostrictive rod (5) is constant even when a change in temperature occurs, to thereby control the amount of actuation of an object to be actuated such that it is constant.